1. Field of the Invention
The present invention relates to a heating device such as an oven which has a greatly improved heat efficiency and a high volumetric ratio in a heating chamber and can be assembled at a high efficiency,
2. Description of the Related Arts
An oven having a self-cleaning function has a heat insulating material having a thickness of 40 to 70 mm formed in the periphery of a heating chamber and thus has a preferable heat insulating performance. An oven having a function of heating food by high-frequency wave depends on forced air-cooling performed by a fan motor because a high-frequency wave-generating device such as a magnetron is incapable of effecting a preferable heat insulation. As a result, the oven has a low heat efficiency and thus, there is a growing demand for the development of an oven having an improved heat efficiency. Even an oven not having the function of heating food by high-frequency wave, volumetric ratio, namely, the ratio of the volume of the heating chamber to the volume of the entire oven is small because a thick heat insulating material is used in the oven. If an oven is designed so that the oven can be accommodated in a cabinet of European size, the width of the heating chamber is 18 inches at most. Thus, it is impossible to manufacture an oven in which the width of the heating chamber is 19 inches. In recent years, a countertop type microwave oven provided with an electric heater has been widely used. But in this type of oven, there is a limitation to the heat insulating construction and thus the oven has a low heat efficiency. Therefore, the improvement of the heat insulating construction is demanded regardless of whether or not an oven has the function of heating food by microwave.
Researches has been carried out with regard to the manufacturing of an oven (not provided with an electric heater) at a low cost and at a high efficiency. Examples of conventional ovens including an oven disclosed in European Patent No. 0110364 are described below. In a microwave oven shown in FIG. 9, a heating chamber 43 and an outer casing 92 are separately constructed, whereas in a microwave oven shown in FIG. 10, walls of the heating chamber 43 serve as an outer casing, and a mechanical chamber 63 is disposed alongside of the heating chamber 43. The oven shown in FIG. 10 is simple in its construction in that walls of the heating chamber 43 serve as the outer casing, i.e., no double construction (wall) is provided, but the operation for installing various electrical parts in the small mechanical chamber 63 is less efficient than the operation required for installing them in the mechanical chamber of the oven shown in FIG. 9 or in mechanical chambers of other conventional ovens. That is, the oven of FIG. 10 is not superior to conventional ovens in productivity. There is a strong and growing demand for microwave ovens which can be manufactured at a high efficiency now that microwave ovens are necessities of life and thus several millions of ovens have been manufactured in the world.